In the early stage, a scanning apparatus is used to scan the image of a single document. After the document has been scanned, the document should be removed from the scanning apparatus and then a next document may be placed on the scanning apparatus in order to be further scanned. Since the process of manually replacing the document is very troublesome, the conventional scanning apparatus is not feasible to scan a stack of documents. Recently, an automatic document feeder is usually integrated into the scanning apparatus. After a stack of documents to be scanned are placed on the sheet input tray of the automatic document feeder, the automatic document feeder will successively transport the documents to perform a scanning operation without the need of manually replacing the documents. This means of automatically feeding the documents is both time-saving and efficient. The automatic document feeder is also feasible to perform a duplex scanning operation.
Generally, the automatic document feeder has a sheet input tray for placing a stack of documents. The automatic document feeder also has a sheet pick-up module for successively feeding the stack of documents from the sheet input tray to the internal portion of the automatic document feeder in a sheet-feeding direction. FIG. 1 is a schematic perspective view illustrating a sheet pick-up module of a conventional automatic document feeder. As shown in FIG. 1, the sheet pick-up module 10 comprises a driving shaft 101, a pick-up driving roller 102, a driving roller sleeve 103, a driving gear 104, a helical driving spring 105, a driven shaft 106, a pick-up driven roller 107, a driven roller sleeve 108, a driven gear 109, a transmission gear 110 and a helical driven spring 111.
The driving shaft 101 is connected to a power device (not shown) for receiving a driving force from the power device, and driven by the driving force. The driving gear 104 is fixedly disposed on the driving shaft 101, and thus the driving gear 104 is synchronously rotated with the driving shaft 101. The driving roller sleeve 103 is arranged beside the driving gear 104 and disposed on the driving shaft 101. The driving roller sleeve 103 is not always rotated with the driving shaft 101.
The helical driving spring 105 is arranged between the driving roller sleeve 103 and the driving gear 104. A first terminal 1051 of the helical driving spring 105 is sheathed by the driving roller sleeve 103. A second terminal 1052 of the helical driving spring 105 is sheathed by the driving gear 104. As such, the helical driving spring 105 is interconnected between the driving roller sleeve 103 and the driving gear 104. The pick-up driving roller 102 is disposed on the driving shaft 101. The pick-up driving roller 102 is not rotated in response to the rotation of the driving shaft 101. The pick-up driving roller 102 is arranged beside the driving roller sleeve 103, and connected to the driving roller sleeve 103. As such, the pick-up driving roller 102 and the driving roller sleeve 103 are synchronously rotated
The transmission gear 110 is engaged with the driving gear 104, so that the transmission gear 110 is synchronously rotated with the driving gear 104. The driven shaft 106 is in parallel with the driving shaft 101. In addition, the driven shaft 106 penetrates through the pick-up driven roller 107, the driven roller sleeve 108 and the driven gear 109. The driven gear 109 is engaged with the transmission gear 110, so that the driven gear 109 is synchronously rotated with the transmission gear 110. The driven gear 109 is fixedly disposed on the driven shaft 106, and thus the driven gear 109 is synchronously rotated with the driven shaft 106.
The driven roller sleeve 108 is arranged beside the driven gear 109, and disposed on the driven shaft 106. The driven roller sleeve 108 is not rotated in response to the rotation of the driven shaft 106. The helical driven spring 111 is arranged between the driven roller sleeve 108 and the driven gear 109. A first terminal 1111 of the helical driven spring 111 is sheathed by the driven roller sleeve 108. A second terminal 1112 of the helical driven spring 111 is sheathed by the driven gear 109. As such, the helical driven spring 111 is interconnected between the driven roller sleeve 108 and the driven gear 109. The pick-up driven roller 107 is disposed on the driven shaft 106. The pick-up driven roller 107 is not rotated in response to the rotation of the driven shaft 106. The pick-up driven roller 107 is arranged beside the driven roller sleeve 108, and connected to the driven roller sleeve 108. As such, the pick-up driven roller 107 is synchronously rotated with the driven roller sleeve 108.
The both terminals of the helical driving spring 105 are respectively sheathed by the driving roller sleeve 103 and the driving gear 104. In addition, the helical driving spring 105 is tightened to apply tightening forces on the driving roller sleeve 103 and the driving gear 104, and thus the helical driving spring 105 is connected with the driving gear 104 and the driving roller sleeve 103. Due to the twisting direction of the helical driving spring 105, the driving shaft 101, the driving gear 104, the driving roller sleeve 103 and the pick-up driving roller 102 are only permitted to rotate in a first rotating direction C1, but fail to be rotated in a second rotating direction C2. As such, when the driving shaft 101 is rotated in response to the driving force from the power device, the driving gear 104, the driving roller sleeve 103 and the pick-up driving roller 102 are synchronously rotated with the driving shaft 101. Whereas, if the driving shaft 101 is not rotated but the pick-up driving roller 102 is rotated in the first rotating direction C1, due to the twisting direction of the helical driving spring 105, the driving roller sleeve 103 that is connected with is rotated with the pick-up driving roller 102 is rotated with the pick-up driving roller 102 in the first rotating direction C1, but the driving gear 104 that is connected with the second terminal 1052 of the helical driving spring 105 is not rotated. The operating principles of the helical driven spring 111 are similar to those of the helical driving spring 105, and are not redundantly described herein.
For feeding plural documents by the automatic document feeder, the sheet pick-up module 10 is firstly contacted with a first document (not shown) which is arranged at the uppermost position of the plural documents. Then, in response to the driving force from the power device, the driving shaft 101 is driven to rotate in the first rotating direction C1, and thus the driving gear 104 fixed on the driving shaft 101 is also rotated in the first rotating direction C1. Upon rotation of the driving gear 104, the driving roller sleeve 103 is also rotated in the first rotating direction C1, and thus the pick-up driving roller 102 is also rotated in the first rotating direction C1. On the other hand, when the driving gear 104 is rotated in the first rotating direction C1, the transmission gear 110 is rotated in the second rotating direction C2 because the driving gear 104 and the transmission gear 110 are engaged with each other. At the same time, the driven gear 109 is rotated in the first rotating direction C1 because the transmission gear 110 and the driven gear 109 are engaged with each other. Upon rotation of the driven gear 109, the driven roller sleeve 108 is also rotated in the first rotating direction C1. As such, the pick-up driven roller 107 that is connected with the driven roller sleeve 108 is also rotated in the first rotating direction C1. In this situation, the first document is fed into the internal portion of the automatic document feeder by the sheet pick-up module 10 at a sheet pick-up speed.
Generally, the automatic document feeder has a feed roller (not shown) for successively transporting plural documents at a feed speed faster than the pick-up speed. In a case that the first document is fed into the internal portion of the automatic document feeder and the front edge of the first document is contacted with the feed roller, the first document is transported by the feed roller at the pick-up speed while the rear edge of the first document is still contacted with the sheet pick-up module 10. Since the feed speed is faster than the pick-up speed, the rear edge of the first document is pulled by the front edge of the first document, and the pick-up driving roller 102 is pulled and rotated by the rear edge of the first document. In this situation, the pick-up driving roller 102 and the driving roller sleeve 103 are synchronously rotated in the first rotating direction C1. Meanwhile, the driving gear 104 is not rotated in order to avoid hindering rotation of the pick-up driving roller 102. After the first document is transported and departed from the sheet pick-up module 10, the sheet pick-up module 10 is contacted with the second document of the plural documents so as to transport the second document.
Due to the helical driving spring 105 and the helical driven spring 111 of the sheet pick-up module 10, the pick-up driving roller 102 and the pick-up driven roller 107 are only permitted to rotate in the first rotating direction C1. Since the pick-up driving roller 102 and the pick-up driven roller 107 fail to be rotated in the second rotating direction C2, the plural documents will not be moved in the direction away from the automatic document feeder. In other words, the helical driving spring 105 and the helical driven spring 111 are very important to the conventional sheet pick-up module 10. However, for overcoming the tightening forces provided by the helical driving spring 105 and the helical driven spring 111, force for driving rotation of the driving roller sleeve 103 and the pick-up driven roller 107 should be sufficiently large. In other words, the driving force provided by the power device should be sufficiently large to operate the sheet pick-up module 10. Under this circumstance, the loading of the automatic document feeder is too heavy.